In its simplest form, Implementation is generally considered by team members as when the project starts. In a phase-controlled project, project team members are only minimally involved prior to the implementation phase. At this point, the scope should be approved and the project is starting in earnest. Implementation is very often the longest phase in the project lifecycle.

From a software perspective the traditionally agreed upon sub-phases include requirements, design, develop/build, and test. These can be extrapolated in many different ways and approached using different methodologies such as Agile, XP or countless others. But, at the core of the project you are managing these four things:

Understanding in a documented way what the end customer needs/wants (Requirements)

Extrapolating the understanding of the requirements into a technical specification to prove that the wants and needs of the customer can be addressed (Design)

Making the requirements a reality (Development/Build)

Test the results for accuracy/completion (Quality Assurance or Testing)